It's not that simple, as it depends on Unicode character properties, rather than something you can achieve readily with PCRE.

Clearly the first step would be to Normalise to NFD so that the diacritics become separate characters,
but there's no simple formula to match "these codepoints are diacritics" other than a complex table extracted laboriously from Unicode data.

What about this: https://en.wikipedia.org/wiki/Perl_Compatible_Regular_Expressions

Unicode character properties

Unicode defines several properties for each character. Patterns in PCRE can match these properties. e.g. \p{Ps}.*?\p{Pe} would match a string beginning with any "opening punctuation" and ending with any "close punctuation" such as "[abc]". Since version 8.10, matching of certain "normal" metacharacters can be driven by Unicode properties when the compile option PCRE_UCP is set. The option can be set for a pattern by including (*UCP) at the start of pattern. The option alters behavior of the following metacharacters: \B, \b, \D, \d, \S, \s, \W, \w, and some of the POSIX character classes. For example, the set of characters matched by \w (word characters) is expanded to include letters and accented letters as defined by Unicode properties. Such matching is slower than the normal (ASCII-only) non-UCP alternative. Note that the UCP option requires the PCRE library to have been built to include UTF-8 and Unicode property support. Support for UTF-16 is included in version 8.30 while support for UTF-32 was added in version 8.32.

It needs to be very carefully thought out with much attention to detail.

- Not just combining classabove but all the various combining classes listed in https://www.compart.com/en/unicode/combining
- And we'd need to consider some of the modifier letters & modifier symbols that also behave as diacritics.

And it it should conform to the character properties in Unicode 11.0 or later, not those of ten years ago.

If the proposed new filter always gives the same results the equivalent convert option in BabelBad, then I'd be very happy.It's always nice to have something that can be scripted.

I think this is covered by properties available in the unicode definition:

ccDiacritic, // Characters that linguistically modify the meaning of another character to which they apply. Some diacritics are not combining characters, and some combining characters are not diacritics.